The present invention is directed to a heat-shrinkable envelope of crosslinked, polymeric plastic material having portions of different degrees of crosslinking to form a tight closure of an article.
U.S. Pat. No. 3,542,077, whose disclosure is incorporated by reference and which was the basis of German Published Application 19 25 739, discloses various crosslinked preforms and a method for the manufacture of the preforms. For example, the preforms are composed of a layer of crosslinked, unmeltable polymer material which is separated from a second layer composed of a meltable, essentially uncrosslinked polymer material by an impermeable barrier layer which is interposed therebetween. Such preforms are shaped as pipes and form a heat-shrinkable structure having an outer heat-shrinkable part and a second inner meltable part that serves the purpose of forming a seal. When the heat-shrinkable part is damaged, for example along an edge, then the shrinking forces acting during the shrinking process can cause continual tearing that can lead to rendering the preform useless. The melting layer that lies below the crosslinkable layer cannot alleviate this situation, since the melting of the material cancels the needed strength. This continued tearing predominantly occurs transversely relative to the shrinking direction.